The ideal storage conditions for platelet concentrates have yet to be defined. Liquid phase storage at either 4 degrees C or 22 degrees C for up to 72 hours are the currently recommended methods for platelet preservation. Factors which may affect platelet yield or viability such as the pH of the suspending medium, centrifuge speed, and drug ingestion by the donor have been extensively studied with platelets stored at each of these temperatures. Investigators at the American Red Cross have recently defined a new "temperature cycled" method of storing platelets, with 4 degrees C storage interrupted every 12 hours with a 45 minute pulse of 37 degrees C storage. In vitro studies indicate that this storage method tends to maximize the advantages while minimizing the disadvantages of each of the standard 4 degrees C or 22 degrees C storage temperatures currently used. The present study is designed to evaluate the in vivo effectiveness of temperature cycled platelets versus 4 degrees C stored platelets in approximately 20 thrombocytopenic patients. Serial platelet counts and bleeding times will be obtained.